Various sorts of grinding wheels have been widely used in diverse of applications with a variety of equipment ranging from simple hand grinders to heavy-load and high-precision grinding machines. Pratically in every application, a grinding wheel during its service or during its high-velocity rotation in abrasive or machining contact with workpieces tends to develop a crack or other damage. This, when it happens, may seriously reduce the machining accuracy and may often cause a serious accident or injury which cannot normally be anticipated. While frequent inspection for damage of a grinding wheel in service and its consequential replacement with a new wheel are thus required, it is quite difficult, if not impossible, to detect such damage with the naked eye, especially a wheel in rotation or during machining.